In a fibre optical communications system it is desirable to be able to control the modulation depth of the light generated by the transmitting laser device. In order to maintain fast switching between states and reduce noise, the transmitting laser is not switched to some defined power and then switched off, but for the low state its output is reduced to a low level. This modulation depth is also described as an extinction ratio (ER), the latter being the ratio of the optical intensity when there is a data ‘1’ and the intensity when there is a data ‘0’. The current required by the laser to deliver these high and low optical outputs is not however constant and indeed is affected by the tolerances between individual laser devices, and also over time, due to the variation of a single laser device's characteristics due to heating and ageing. Such variations can occur in normal operation as a device heats up in use.
Hence it is desired not only to be able to compensate automatically for manufacturing tolerances and parameter drift in the laser itself, but also to be able to reach some defined target modulation level where the data pattern has a random characteristic with only limited low frequency content.
There are several methods for controlling the modulation (or ER) are described in prior art. Usually these make assumptions about characteristics of the devices or data patterns that may not always be valid. Many methods are related to a technique presented by Smith (Electronics Letters, October 1978 pp 775-776), wherein a low amplitude low frequency (LF) modulation is added to the normal laser current. The fluctuations in the optical output from the laser at the known LF modulation frequency permit the estimation of the zero and/or average optical levels, and hence the slope of the laser current/optical output characteristic can be calculated. Because this modulation is relatively low frequency, the system provided to monitor the optical output power does not have to have a high bandwidth, which is an attractive feature. Provided that the laser characteristic does not have excessive non-linearity, it is thus possible to construct a feedback loop to maintain reasonable control over the modulation depth (or ER) provided that the feedback loop implied in this system has time to settle, as in the case of continuous mode operation. The main limitation with such systems is that there is an implied assumption that the laser's current vs optical output characteristic is substantially linear. At higher operating power levels this is not a safe assumption.
A more direct approach is to attempt to measure the optical output levels that directly correspond to logical 1 and logical 0 states. There is normally provided a photodiode to sense the optical output power from the laser, together with an associated monitor transimpedance amplifier, together hereinafter referred to as a monitor channel. The performance of this latter function places restrictions on the operation of any such control loop, since for cost and power reasons, the bandwidth of any monitor channel used to control the system is frequently much less than that of the main communication channel.
The transmitted optical data will switch between its logical 1 and logical 0 levels at rates defined by system level requirements, and will remain substantially constant at these levels for the duration of the number of consecutive symbols of the same sign. This consecutive number is referred to as the run length. In many practical systems, the monitor channel bandwidth is sufficiently restricted as to cause its own output to settle only if the observed optical signal is constant for a relatively large number of symbols. Given that in a random data stream the probability of a given run length decreases as the length increases, it is clear that a monitor channel of restricted bandwidth will give only very infrequent outputs corresponding directly to the optical 1 and optical 0 levels.
In some prior art, attempts are made to achieve an accurate estimation of the logical 1 and 0 levels by gating the output of the monitor channel such that its value is only considered when a long run length is detected in the incoming data stream and hence the value observed via the monitor channel will have had time to settle. This approach has some merit, but it still places significant demands on the bandwidth of the monitor channel as a ratio of the symbol rate, with attendant increased power consumption likely in the monitor channel.
Hence some other methods are sought by which the optical 1 and 0 levels can be estimated or inferred from the outputs of a monitor channel having restricted bandwidth compared with the symbol rate.
Rather than consider the direct time-domain output from the monitor channel, one may instead look at the statistics of the monitor signal, and in particular at the probability density function (PDF) of this output. Consider the situation if the monitor channel were to have unlimited bandwidth. Since the optical output has a defined time to change between levels, and then remains at each level for the run length at some instant, it will be apparent that the PDF will have a bi-modal form, with a near constant level between the two peaks. For a random data signal, the relative magnitudes of the peaks at each end of the PDF and the level in between will vary with the maximum run length used. As the bandwidth of the monitor channel is reduced below approximately 10% of the symbol rate, the bimodal form is lost and becomes more Gaussian, and the values corresponding to the ideal logical 1 and logical 0 are not very evident in the PDF.
If the bandwidth is reduced still further below approximately 5% of the symbol rate, the tails of the Gaussian form of the PDF drop to near zero at the expected logical 1 and 0 values. However, the mean and standard deviation values do not depend strongly on the maximum run length of the data stream. The main determining factors for the standard deviation (relative to the mean value) are the ER of the optical signal and the bandwidth of the monitor channel. Hence if the bandwidth of the monitor channel can be accounted for in the measurement system, the standard deviation of the monitor output may be used to infer the ER.
Measuring the bandwidth of the monitor channel directly is possible but not very convenient in a complete optical system as it depends on the capacitance of the photodiode used for the monitor function. It is also necessary to have knowledge of the absolute gain through the monitor channel so that the standard deviation observed can be appropriately scaled. An alternative and more practical method is to use a parallel replica signal path, whereby the effects of the gain and bandwidth on ideal data may be taken into account.